The Magnus effect isn't a significant player in bullet ballistics unless there is also a crosswind (this is discussed further down in the Wikipedia article you referenced.) In fact, this would be an argument *against* using rifled barrels, since eliminating the bullet spin would negate the Magnus effect of any crosswind present.

You're correct about the location of the center of pressure versus the center of gravity and its effect on stability, but both the center of pressure and center of gravity are independent of spin *for a perfectly spherical projectile*.

Also, while forward velocity drops off faster than rotational velocity, that's not really relevant - that's just because forward velocity is affected by drag from the relatively-large frontal area of the bullet, while rotational drag just comes from the interaction of the thin boundary layer with the atmosphere.

I'll wait until I've gotten a copy of the article I mentioned earlier and worked through the derivations for the full Greenhill formula, but the more I look into it, I think the only reason that spherical projectiles benefit from rifling is that as an earlier poster noted, the projectiles aren't really round (or smooth).